The invention relates in general to antenna structures in radio apparatuses. In particular the invention relates to a planar inverted-F antenna (PIFA) structure that has two resonating frequencies.
FIG. 1 shows a known basic model of a planar inverted-F antenna 100 comprising a planar electrically conductive radiating element 101, electrically conductive ground plane 102 parallel to said radiating element, and, connecting these two, a ground contact 103 which is substantially perpendicular to the radiating element and ground plane. The structure further includes a feed electrode 104 which also is substantially perpendicular to the radiating element and ground plane and which can be coupled to an antenna port (not shown) of a radio apparatus. In the structure of FIG. 1 the radiating element 101, ground contact 103 and the feed electrode 104 are usually manufactured by cutting a thin metal sheet into a suitable rectangular shape which has got two protrusions bent to a right angle. The ground plane 102 may be composed of a metallized area on the surface of a printed circuit board so that the ground contact 103 and feed electrode are easily connected to holes on the printed circuit board. The electrical characteristics of the antenna 100 are affected in general by the dimensions of its elements and in particular by the size of the radiating element 101 and its distance from the ground plane 102.
A disadvantage of the antenna structure depicted in FIG. 1 is its poor mechanical sturdiness. Various solutions have been proposed to this problem. European Patent document No. 484,454 discloses a PIFA structure according to FIG. 2 wherein a radiating element 201, ground plane 202 and a ground contact 203 connecting these two are realized as metal platings on surfaces of a solid dielectric body 204. The antenna is fed through a coupling element 205 which does not touch the radiating element 201. An electromagnetic coupling exists between the coupling element 205 and radiating element 201, and the coupling element extends over the edge of the dielectric body 204 to a point that can be coupled to the antenna port of a radio apparatus. The structure is mechanically sturdy, but the dielectric body block makes it rather heavy. Furthermore, the dielectric body makes the impedance bandwidth of the antenna narrower and degrades the radiation efficiency as compared to an air-insulated PIFA structure.
The radiating element of a planar inverted-F antenna need not be a simple rectangle as in FIGS. 1 and 2. FIG. 3 shows a known PIFA radiating element 301 design. The rectangular shape is broken by a gap 302 which forms a sort of strip in that portion of the radiating element which is farthest away from the feed point 303 and ground contact 304. The purpose of the gap usually is to increase the electrical length of the antenna and thus affect the antenna""s resonating frequency.
All the PIFA structures described above are designed such that they have a certain resonating frequency as well as an operating frequency band centering round said resonating frequency. In some cases, however, it is preferable that the antenna of a radio apparatus have two different resonating frequencies. An example of such a case is a cellular radio system terminal which has to be capable of operating in two different cellular radio systems or in two different frequency ranges of a single cellular radio system. The difference of the frequencies may be considerable as at the moment of writing this patent application the frequency areas of currently existing cellular radio systems range from about 400 MHz to about 1900 MHz, and it is probable that even higher frequencies will be taken into use in the future.
FIGS. 4a and 4b show dual-frequency PIFA radiating elements known from the publication xe2x80x9cDual-Frequency Planar Inverted-F Antennaxe2x80x9d by Z.D. Liu P.S. Hall, D. Wake, IEEE Transactions on Antennas and Propagation, Vol. 45, No. 10, October 1997, pp. 1451-1457. In FIG. 4a the antenna comprises a rectangle-shaped first radiating element 401 and a second radiating element 402 surrounding said first radiating element from two sides. The first radiating element has got a feed point 403 and ground contact 404 of its own, and the second radiating element has got those of its own, 405 and 406. In FIG. 4b the antenna comprises a continuous radiating element 410 which is split into two branches by a gap 411. The feed point 412 is located near the inner end of the gap 411 so that it can be said that the branches have different directions from the feed point on. Both branches have electrical lengths of their own which differ from each other considerably. The ground contacts 413 are located near the edge of the structure.
It is further known a dual-frequency PIFA radiating element 501 according to FIG. 5 which has got two branches in the same manner as the radiating element in FIG. 4b. In FIG. 5, the outermost ends of both branches extend to the edge of the printed circuit board, depicted by the broken line, which supports the radiating element. This structure provides a somewhat wider antenna impedance band, i.e. frequency range around a particular resonating frequency in which the antenna impedance matching to the antenna port of the radio apparatus is good. At the same time, however, the SAR value, which represents the amount of radiation absorbed by the user, becomes rather high, especially in the higher frequency band.
An object of the present invention is to provide a planar antenna with at least two resonating frequencies. Another object of the present invention is that the planar antenna according to it can be tuned in a versatile manner. Yet another object of the invention is that the antenna according to it has a relatively low SAR value.
These and other objects of the invention are achieved by a planar antenna structure which has an outer branch and an inner branch such that the outermost end of the inner branch is for the most part surrounded by the outer branch.
The planar antenna according to the invention comprises a planar radiating element formed of a conductive area confined within a substantially continuous border line, said conductive area being split by a non-conductive gap which divides the planar radiating element into a first branch and second branch such that both the first and the second branch have an outermost end, and which has a head end at said sub-stantially continuous border line and a tail end within the conductive area. The planar antenna according to the invention is characterized in that at its head end the gap has a certain first direction and at another point of the gap it has a certain second direction which differs more than 90 degrees from the first direction when the directions are defined from the head end to the tail end of the gap, whereby the outermost end of the second branch, confined by the gap, is located within the continues border line, surrounded by the first branch.
The invention is also directed to a radio apparatus. It is characterized in that it comprises a planar radiating element like the one described above and a ground plane which is substantially parallel to said radiating element and located with respect to the planar radiating element such that in the typical operating position of the radio apparatus it is between the planar radiating element and the user of the radio apparatus.
The planar antenna according to the invention comprises a planar radiating element split into at least two branches by a gap. The electrical lengths of the branches are chosen such that the first branch efficiently operates as an antenna at a first operating frequency of the structure and, respectively, the second branch efficiently operates as an antenna at a second operating frequency of the structure. An advantageous method is to choose the electrical lengths such that the electrical length of each branch corresponds to a quarter of a wavelength at the desired operating frequency. The feed point and ground contact(s) of the antenna are preferably located near the point where the branches come together.
In order to minimize the SAR value the outermost end of the second branch is located such that it is not by the edge of the planar radiating element but is substantially surrounded by the first branch. It has proven advantageous that the second branch then is the branch corresponding to the higher operating frequency. The layout is brought about by shaping the gap at least in some parts strongly curvilinear so that the outermost end of the second branch remains on the concave side of the curved portion of the gap.
The electrical characteristics of the antenna structure strongly depend on the width and shape of the gap. It is usually advantageous to have rather a narrow gap so that the branches function as capacitive loads to each other. Capacitive loading decreases the resonating frequencies so that an antenna intended for certain particular frequency ranges can be made smaller than without said capacitive loading. In addition, the location and shape of the gap affects the ratio of the resonating frequencies of the antenna, as well as the bandwidth in both resonating frequency ranges.
In accordance with a preferred embodiment of the invention the gap is shaped such that at least the branch corresponding to the lower resonating frequency gets wider either in steps or steplessly towards its outermost end. A branch that gets wider towards its outer end facilitates a smaller radiating element without considerably compromising the radiation or impedance bandwidth.